The objective of this program is to improve the reliability of a recently developed bioassay for human tumor stem cells. This assay is currently being used to study the biological properties of human tumor cells and to predict the efficacy of cancer chemotherapeutic agents. Currently, the low cloning efficiencies of many tumor cells in this system preclude the widespread clinical application of this assay. Therefore, this study is exploring improvements in growth conditions for tumor cells. Media conditioned by long term cell lines is being used to stimulate the growth of primary tumor cells in soft agar. The use of purified growth factors and hormones is being evaluated. The effect of non-tumor cells on tumor cell growth is also under study. The tumor stem cell assay has proven useful both in identifying promising new anticancer agents and for predicting an individual patient's sensitivities to anticancer drugs. The standardization of this bioassay could lead to its use as a predictive tool in cancer chemotherapy and anticancer drug screens.